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 Title: Determination Of The Ionization Energy Of The Metastable 2 1s0 State Of 4he Through Rydberg-series Extrapolation Author(s): Clausen, Gloria Contributor(s): Merkt, Frédéric; Schmutz, Hansjürg; Agner, Josef A.; Jansen, Paul Subject(s): Mini-symposium: Precision Spectroscopy for Fundamental Physics Abstract: \noindent Helium, as one of the simplest few-electron atoms, is well suited to test fundamental interactions and QED calculations with a high precision. Its importance is underlined by recent advances in the determination of particle properties such as the $\alpha$-particle charge radius\footnote{J. Krauth et al., Nature 589, 527531 (2021)}. Yet, the most accurate theoretical\footnote{K. Pachucki, V. Patk\'o$\check{\mathrm{s}}$, and V. A. Yerokhin, Physical Review A 95, 062510 (2017)}$^,$\footnote{G. W. Drake and Z. C. Yan, Canadian Journal of Physics 86, 45 (2008)} %\footnote{ V. Patk\'o$\check{\mathrm{s}}$, V. A. Yerokhin, and K. Pachucki, Physical Review A 103, 012803 (2021)}$^,$ and experimental\footnote{W. Lichten, D. Shiner, and Z.-X. Zhou, Physical Review A 43, 1663 (1991)}$^,$\footnote{C. J. Sansonetti and J. D. Gillaspy, Physical Review A 45, R1 (1992)} data on the ionization energies of the low-lying electronic states show a significant discrepancy of more than 2 $\sigma$ (about 3 MHz). We report on the determination of the ionization energy of the metastable 2 $^1\mathrm{S}_0$ state of helium through Rydberg-series extrapolation with a relative uncertainty of $5 \times 10^{-11}$ through the determination of 20 different $n\mathrm{p} \leftarrow2\;^1\mathrm{S}_0$ transition frequencies in the range of $n$ from 24 to 102, yielding quantum-defect parameters for the $n$p Rydberg states. A one-photon excitation scheme was employed, using the frequency-doubled output of a narrowband cw laser source (312 nm), calibrated to a frequency comb referenced to a GPS-disciplined Rb clock. The major sources of systematic uncertainties are minimized by (i) carrying out the experiment in a doubly skimmed, pulsed supersonic beam of metastable $^4$He atoms to minimize the Doppler broadening, (ii) compensating electric stray fields to below 0.7 mV/cm, and (iii) cancelling the first-order Doppler shift by monitoring two Doppler components resulting from two counter-propagating laser beams. The effects of residual stray electric fields are included as DC-Stark shifts in the analysis. Our new value of the ionization energy of the $2\;^1\mathrm{S}_0$ state of $^4$He, with an absolute uncertainty better than 50 kHz, represents an improvement by a factor of about five in precision. It also allows for a purely experimental determination of the ionization energies of several other low-lying electronic states of $^4$He, which were previously only accurately known through combination of measured and theoretical energy intervals. Issue Date: 2021-06-23 Publisher: International Symposium on Molecular Spectroscopy Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/111291 Date Available in IDEALS: 2021-09-24
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